Classical Phenylketonuria (PKU) is a recessive monogenic disorder in amino acid metabolism that results from a deficiency of hepatic phenylalanine hydroxylase (PAH), and it predisposes affected individuals to severe and permanent mental retardation. While the disease phenotype in PKU patients can be prevented by dietary restriction, the treatment suffers from the need of life-long patient management and poor patient compliance. Discontinuation of dietary restriction during pregnancy in female PKU patients has also caused various birth defects in the offspring regardless of the PAH genotypes. This syndrome, known as maternal PKU, has become a public health concern since most treated female PKU patients of child-bearing age have been off diet restriction. We have therefore begun the development of complementary treatment modalities for PKU and maternal PKU by genetic reconstitution in vivo. To achieve persistent and high level transgene expression in the liver, recombinant adeno-associated virus (AAV) vectors deleted of all viral genes will be used for in vivo delivery of the murine PAH ene to the livers of the PAH-deficient Pah anu2 mice, an extraordinarily faithful animal model of PKU and maternal PKU in humans. Recombinant AAV vectors have recently been shown to effectively transduce hepatocytes in vivo, which resulted in the persistent expression of transgene products without apparent toxicity or eliciting a cellular immune response against the virally transduced cells. Its application in treatment of metabolic disorders however, is limited by the fact that only a small fraction of vector-transduced hepatocytes in vivo resulted in persistent expression of the transgenes, thus requiring high levels of transgene expression per cell for the treatment to be effective. In addition, there still remains the difficulty in large-scale production of purified rAAV vectors for effective transgene delivery into large animals including humans. To overcome or circumvent these obstacles, we propose the following molecular strategies: 1) develop and use of hybrid rAAV vectors with capsids from alternative serotypes to enhance hepatocyte transduction efficiency in vivo; 2) incorporate cis-acting genetic elements into the promoter-enhancer region of transgene expression cassettes that will lead to elevated gene transcription; and 3) insert cis-acting elements into the 3'-untranslated region of mRNAs that will increase their stability in vector-transduced cells. We envision that these molecular strategies for hepatic gene transfer and expression will be synergistic in elevating the level of mPAH activity in the livers of PKU mice, which will restore blood phenylalanine levels to normal. This beneficial outcome might also be accomplished at reduced vector doses so that the need for large-scale production of the purified vectors can be minimized. Successful conduct of these studies will provide the scientific foundation for future applications in the genetic treatment of PKU as well as other metabolic disorders secondary to hepatic enzyme deficiencies in patients.